Ramdom invoice number in woocommerce using WooCommerce Print Invoice & Delivery Note plugin.

random-invoice-number-in-woocommerce.php

<?php

//woocommerce-delivery-notes/includes/class-wcdn-print.php
public function get_order_invoice_number( $order_id ) {

// Custom random invoice no
$invoice_count = mt_rand(1,9999);
$invoice_prefix = mt_rand(1,9999);
$invoice_suffix =  mt_rand(1,99999);

// Check if this invoice no is already exists, if exists then create new invoice no
$custom_invoice_no =  $invoice_prefix .'-'. $invoice_count .'-'. $invoice_suffix;
// Add the invoice number to the order when it doesn't yet exist
$meta_key = '_wcdn_invoice_number';

//Check if any order exists with this invoice id
$args = array(
  'post_type'   => 'shop_order',
  'meta_query'  => array(
    array(
      'key' => '_wcdn_invoice_number',
      'value' => $custom_invoice_no
    )
  )
);
$my_query = new WP_Query( $args );
while( !empty($my_query->have_posts()) ) {
  // Order exists create new invoice id
  $invoice_count = mt_rand(1,9999);
  $invoice_prefix = mt_rand(1,9999);
  $invoice_suffix =  mt_rand(1,99999);

  $custom_invoice_no =  $invoice_prefix .'-'. $invoice_count .'-'. $invoice_suffix;
  $args = array(
    'post_type'   => 'shop_order',
    'meta_query'  => array(
      array(
        'key' => '_wcdn_invoice_number',
        'value' => $custom_invoice_no
      )
    )
  );
  $my_query = new WP_Query( $args );
}

$meta_added = add_post_meta( $order_id, $meta_key, $custom_invoice_no, true );
// Update the total count
if( $meta_added ) {
  update_option( 'wcdn_invoice_number_count', $invoice_count + 1  );
}

// Get the invoice number
return apply_filters( 'wcdn_order_invoice_number', get_post_meta( $order_id, $meta_key, true ) );
}